U.S. Pat. Nos. 3,896,131 and 3,985,882 described benzo[ij]quinolizine-2-carboxylic acids which are useful antimicrobial agents. The prior art compounds may contain various substituents in the 8, 9 or 10-position including amino (--NH.sub.2),N,N-dimethylamino [(CH.sub.3).sub.2 N--], alkanamido and trifluoroacetamido groups. However, prior to the present invention, it was not known that compounds substituted in the 8-position with five-membered-nitrogen-containing heterocyclic nuclei exhibit useful antimicrobial activity.